"System and method for checking the quality of a hammering operation on a mechanical component during the hammering operation"
The present invention relates to a system and a method for checking the quality of a hammering operation on mechanical components, during the hammering operation. The expression "hammering operation" in the present description and in the claims is used to indicate an operation by which a component is pressed into a cooperating seat until an interference fit coupling is achieved. A possible application of the system and method according to the invention lies in checking the quality of the assembling operation of the seating rings for the valves of an internal combustion engine, during assembling of the engine cylinder head by means of automated tools.
During the hammering operation of a mechanical component, such as the hammering of a ring of the above described type within a seat provided therefor on the lower surface of the cylinder head of an internal combustion engine, various inconveniences may arise. A first possible problem takes place when the component to be hammered is out of tolerance, i.e. when the interference between the component and the seat therefor is greater than the maximum interference permitted by the tolerances of the coupled elements or lower than the minimum theoretical interference. In the first case, the excessive force generated by the high interference may prevent the ring from coming fully into abutment against its seat. Therefore, a gap, even of small dimension (0.01 mm), may remain between the ring and the seat which gives rise to malfunctions during use of the engine. In the latter case, the hammering force is very low so that the ring may come out of its seat during operation of the engine, due to the mechanical and thermal stresses.
In general, the above described problem can be solved easily by monitoring the value of the hammering force to which the component is subjected during its movement in the hammering operation. If this force is greater or lower than the values respectively corresponding to the maximum interference coupling and the minimum interference coupling, the fit coupling is to be considered unacceptable. Devices for controlling the quality of the hammering operation which are able to carry out this analysis have been already known and used.
A second type of problem takes place in the case of a rupture of the component during the hammering operation. This problem is to be considered similar to the one previously described, since the rupture of the component during the hammering operation implies an immediate decrease of the interference between the component and its seat and hence an abrupt decrease of the hammering force during the central portion of the travel of the component. This type of problem, which can cause clearly problems on the engine, can be easily detected by an analysis of the value of the hammering force during the travel for obtaining the hammering operation, by means of the known devices which are already in use.
However, the devices for controlling the quality of the hammering operation which are presently used are not able to detect the existence of further types of problems reliably. One of these problems is the one due to the presence of foreign bodies, typically machining chips, within the seat for receiving the component. The presence of these chips prevents the component, such as a valve ring, from coming into abutment against the bottom of its seat throughout the whole circumference of the seat. Also this type of defect may affect the operation of the engine. The presence of chips within the seat implies a variation of the force opposing hammering of the component during the end stage of the hammering stroke (about 3-4 hundredths of millimetre before the component reaches the end position in abutment against the seat bottom). There is no way to detect this defect by a checking operation of conventional type, by considering maximum admissible values of the force calculated as an absolute function of the displacement, since this defect takes place within a range, with respect to the absolute position during hammering, which is comprised within the admissible positioning tolerances of the element. For instance, for cylinder heads to which the device according to the invention has been applied, the plane of the abutment surface of the seat has a positioning tolerance, with respect to the reference plane of the head within a clamping tool, of about +/-1.3 mm.
A further type of defect, which is perhaps even more difficult to be detected, is that due to an angular positioning error between the component and its seat. Also in this case, the consequence of a non proper hammering may imply, for instance in the above mentioned case of valve rings, an engine malfunction. Also in this case, one could try to detect a variation of the force opposing hammering in the last hundredths of mm of the travel of the component before the latter comes into abutment against the bottom of its seat. The hammering force in the central portion of the travel, as an average, is higher with respect to the case of perfect alignment, but not necessarily greater with respect to the case of a high, but yet admissible, interference. Therefore, it is not possible to distinguish this defect by analysing the value of the average hammering force during the component travel, since it is necessary, as in the previous case, to carry out a careful analysis on the end portion of the travel. The variation of the value of the force opposing hammering during the end portion of the travel is to be attributed to the mechanical yield of the manipulating robot which is usually used to carry out the hammering operation, which robot tries to adapt its position to the geometric shape of the seat, when the latter is a little misaligned with respect to the theoretical position.